Process for the continuous heat treatment of a low alloy steel wire material

ABSTRACT

A PROCESS FOR CONTINUOUSLY HEAT TREATING A WIRE, ROD, BAR AND STRIP MATERIAL MADE OF LOW ALLOY STEEL SUCH AS A CHROMIUM-VANADIUM, CHROMIUM-SILICON AND BORON STEEL, CHARACTERIZED BY HEATING THE MATERIAL TO A TEMPERATURE NOT LOWER THAN ITS A3 TRANSFORMATION POINT, COOLING THE HEATED MATERIAL AT A SUITABLE COOLING RATE NOT LOWER THAN ITS LOWER CRITICAL COOLING RATE, THE FORMER COOLING RATE BEING SUITABLY DETERMINED DEPENDING ON THE KIND OF STEEL AND THE DIAMETER OR THICKNESS OF THE MATERIAL, TO A TEMPERATURE RIGHT ABOVE ITS MS POINT, AND THEN REHEATING THE COOLED MATERIAL TO A TEMPERATURE NO HIGHER THAN ITS A1 TRANSFORMATION PONT, THEREBY RENDERING THE MATERIAL MAINLY FINE-PEARLITIC IN STRUCTURE; AND A LAOW ALLOY STEEL TREATED BY THE PROCESS.

May 30, 1972 PROCESS FOR THE CONTINUOUS HEAT TREATIIENT AKIRA NAKAGAWAETAL OF- A LOW ALLOY STEEL WIRE MATERIAL Filed Jan. 21. 1969 FORMATIONPOINT(MS) Y 400 MARTENSITE TRANS- I LL! I! 3 4 0: l-Ll '0. E ii --c0o|ms RATE CURVE ---REHEATING CURVE TIME (SECOND) Fig.l

United' States Patent-Oflice 3,666,572 Patented May 30, 1972 Int. 01.621.1 1/18 US. Cl. 148-134 Claims ABSTRACT OF THE DISCLOSURE A processfor continuously heat treating a wire, rod, bar and strip material madeof low alloy steel such as a chromium-vanadium, chromium-silicon andboron steel, characterized by heating the material to a temperature notlower than its A transformation point, cooling the heated material at asuitable cooling rate not lower than its lower critical cooling rate,the former cooling rate being suitably determined depending on the kindof steel and the diameter or thickness of the material, to a temperatureright above its Ms point, and then reheating the cooled material to atemperature not higher than its A; transformation point, therebyrendering the material mainly fine-pearlitic in structure; and a lowalloy steel treated by the process.

This invention relates to a novel process for heat treating suchmaterials as wires, rods, bars or strips made of a steel containing, onthe basis of weight, 0.l50.65% carbon, at least one of up to 2.20%silicon, up to 2.05% manganese, 0.20-2.70% chromium, (US-3.80% nickel,0.l52.10% molybdenum, 0.70l.20% aluminum, 050 150 tungsten, 0.10-0.50%vanadium, up to 0.006% boron, up to 0.05% niobium, up to 0.15% titaniumand up to 0.15% zirconium, the balance being iron and incidentalimpurities; said process comprising cooling the materials such as thewires, rods, bars or strips to a temperature immediately above the Mspoint thereof at a suitable cooling rate corresponding to the kind ofsteel the materials are made of and the diameter or thickness of thematerial and then reheating the thus-cooled materials. Moreparticularly, this process is characterized by, while passing thematerials continuously through a heat-treating apparatus, heating thematerials at a temperature above their A trans-formation point for asuitably short time, this time being determined by calculating on thebasis 0.10-2.0 minutes per mm. of diameter or thickness of the material,cooling the thus-heated materials to a temperature immediately abovetheir Ms point at a cooling rate not lower than their lower criticalcooling rate, this rate varying with the kind of steel the materials aremade of and the diameter or thickness of the materials and thenreheating the materials so cooled to a temperature below their A,transformation point, thereby allowing the materials to become mainlyfine pearlite in structure.

Wires, rods, bars and strips made of such low alloy steel, when treatedby the conventional heat treating processes, have a low strength becausethey are annealed in a bundle or coil and have a poor cold workabilitybecause they become spheroidal cementite (the carbide beingspheroidized) or coarse pearlite in structure.

The following Table 1 shows the results obtained from the test in whichwire materials made of a chromiumvanadium (Cr-V) alloy steel and heattreated in the conventional manner, were drawn into finer wires whichwere then tested for tensile strength, reduction of area and elongation.The table shows that some of the finer wires so obtained, which arecoarse pearlite in structure, remarkably increase in toughness and areunable of being further drawn when drawn to such an extent that areduction of area of approximately 60% is reached, and that theremaining finer wires, which are spheroidal cementite in structure, alsobehave in this manner when drawn to an extent that a reduction of areaof nearly 67% is obtained.

On the other hand, the process of this invention is different fromconventional processes and comprises continuously heat treating wires,rods, bars and strips to provide them with superior subsequentworkability although they can have excellent toughness and strengthwithout any other treatments after the heat treatment.

The conventional processes for the continuous heat treatment of wiresinclude a patenting process for the manufacture of cold-drawn wires suchas music spring steel wires and other hard drawn wires, an oil quenchingand tempering process for the manufacture of various oil tempered wires,and the like. Of those conventional processes, the patenting process canbe applied only to carbon steel wires and it comprises heating a carbonsteel wire to a predetermined temperature above the A transformationpoint thereof and then allowing the thus-heated steel to cool in the airor keeping it in a bath of molten lead to effect its semi-isothermaltransformation. And the oil quenching and tempering process comprisesheating a wire likewise to a temperature above the A transformationpoint thereof, quenching the heated wire in oil and then tempering thequenched Wire at a predetermined temperature. In these processes, wiresin a coil being heat-treated are placed on the pay-off stand of a heattreating apparatus, thereafter pulled at their end to pass them throughthe heat treating unit of the apparatus and then wound on a take-upblock.

Now, the process of this invention comprises heating low alloy steelwires, rods, bars and strips to a temperature above the A transformationpoint thereof, thereafter cooling the heated materials at a cooling ratenot lower than the lower critical cooling rate thereof to a temperatureimmediately above the Ms point thereof by using suitable means and thenreheating the cooled materials to a temperature not higher than the Atransformation point thereof. This is accomplished, as in the case ofthe wires treated by the patenting and the continuous oil quenching andtempering process previously mentioned, by heating, cooling andreheating the starting wires in a continuous way, and then rewinding thethustreated wires on a take-up block, while introducing the startingwires on the pay-off stand to a heat treating apparatus. The process ofthis invention has advantages in that it requires a shorter time to heattreat materials to be treated and can treat several tons of materialssuch as wires simultaneously thereby rendering itself very economicaland providing the materials with superior mechanical properties and coldworkability.

The process of this invention is more particularly de scribed asfollows:

It comprises austenitizing a starting low alloy steel material in ashort time, continuously cooling at a suitable cooling ratecorresponding to the kind and diameter or thickness of the material to atemperature immediately above the Ms point thereof by, for example,allowing the austenitized material to cool in the air or subjecting itto cooling with forcibly circulated air or with heated air so that thecooled material has a singleor multiphase structure such as supercooledaustenite, supercooled austenite-bainite, supercooledaustenite-bainite-ferrite, supercooledaustenite-bainite-ferrite-pearlite or the like, and then reheating thecooled material to a temperature not higher than the transformationpoint A thereof to transform the supercooled austenite to fine pearliteor the coexisting bainite to secondary pearlite. The structure thusobtained is fine perlite and is dilferent from spheroidal cementite,coarse pearlite and the like, and the material having the fine pearlitestructure is superior not only in toughness, strength and severeworkability but also in capacity of being cold-drawn or cold-rolled,thereby allowing it to be drawn into a material having a smallerdiameter or thickness with less frequent intermediate heat treatments.

Of alloy steels which may be treated by the process of this invention,Cr-V steel, Cr-Si steel, boron steel, Mi-Mn steel and Mn-Cr steel areshown with the range of composition thereof in Table 2.

This invention will be better understood by the following examples.

EXAMPLE 1 A Cr-V steel wire having a diameter of 9 mm., was continuouslypassed through a heat treating apparatus, and was simultaneously heatedto a soaking temperature of 920 C. higher than its A transformationpoint and maintained at this temperature for 2.2 minutes, cooled in theair to a temperature of 455 C. immediately above its Ms point, reheatedto a temperature of 550 C. below its A transformation point for a periodof 2.8 minutes and then allowed to cool in the air. The wire so treatedaccording to this invention was cold drawn to the extent that itsoriginal diameter 9.0 mm. was reduced finally to 2.9 mm., during whichtest pieces having the respective diameters of 7.4, 5.7, 4.1, 3.7 and2.9 mm. were sampled. The samples were then tested for tensile strength,reduction of area, elongation and Vickers hardness; and the remainder ofthe treated wire, which was 9 mm. in. diameter and was not subjected toany further treatments such as drawing after the heat treatment, wasthen tested in the same manner as above. The particular chemicalcomposition of the wire used, the operational conditions for heattreating it, and the relationship between the change in diameter andthat in mechanical properties thereof. are respectively shown in Tables3, 4 and 5.

EXAMPLES 2, 4, 5 AND 6 In these examples the same heat-treatmentprocedure was followed as in Example 1, but using Cr-Si steel. boronsteel, Si-Mn steel and Mn-Cr steel wire samples the chemicalcompositions of which are respectively shown in Table 3 and under theoperational conditions respectively shown in Table 4. The wire samplesso heat treated were further treated in quite the same way as shose inExample 1 to obtain mechanical properties which are shown in Table 5.

Table 5 shows that in spite of the heat treatment for a shorter periodof time, the heat-treated wire samples which had been drawn to theextent that the reduction of area thereof reached the values betweenapproximately 87 and 93% at the end of the cold drawing, still indicated4 the final reduction of area of 20 45% and elongation of 1.5-3.0% whichrepresent excellent cold workability.

EXAMPLES 7-12 This invention can also be applied to the heat treatmentof wires having a smaller diameter. This was confirmed and proved trueby heat treating the wire samples, the chemical compositions of whichare respectively shown in Table 3, under the conditions shown in Table6, cold drawing the heat treated samples at ambient temperatures,testing the drawn samples for mechanical properties which are shown inTable 7 and then comparing the results of this table with those of Table5.

The effects of the heat treatment of this invention will also beillustrated by the following examples.

EXAMPLES 13-18 In these examples, wire samples of Cr-Si steel having thesame composition as the one shown in Table 3 were heat treated under theconditions indicated in Table 8. The mechanical properties of the wiresamples thus treated are also shown in Table 8, from which it is seenthat all the starting samples have each come to have excellent capacityof being drawn after the heat treatment at the different temperature notlower than the transformation point thereof during the different periodof treating time. It is also seen from Table 8 that the soakingtemperature and time which may practically be employed in this case, arethe temperature from the A transformation point of Acm transformationpoint up to approximately 950 C. and the time of nearly two minutes per1.0 mm. of diameter of the wire to be treated, respectively, that theuse of temperatures and times exceeding the aforesaid ones will renderthe process less economical and tend to cause decarbonization, othersurface defects and the like of the wire and that such treated wires asshown in this table may find their use in the industrial field becausethey even have superior mechanical properties without further treatmentsuch as cold working.

FIG. 1 is a diagram showing the continuous cooling transformation of aCr-Si steel wire, of a 4 mm diameter which contains 0.54% carbon, 1.43%silicon, 0.53% manganese, the balance consisting substantially of ironand which has been heat treated at a temperature of 900 C. for 5 minutesbefore cooling; and FIG. 2 is an enlarged photograph (magnification:8000) showing the fine pearlite structure of a Cr-Si steel wire heattreated by the process according to this invention, which processcomprises heating a starting Cr-Si steel wire of a 4 mm. diameter havingthe same composition as the Cr-Si steel in Table 3 at 880 C. for 1.6minutes, air-cooling the heated wire to 375 C. and then reheating thecooled wire at a soaking temperature of 600 C. for 2 minutes to producesaid heat-treated Cr-Si steel wire.

EXAMPLES l930 In the examples, test pieces of each of a Cr-Si steel wireand a Cr-V wire, the wires being 4.5 mm. in diam eter, were heat treatedunder the same conditions except that they were reheated at therespective soaking temperatures different from one another within therange of 650 C. to 400 C., as shown in Table 9, in order to find thevariation of the thus-treated test pieces in mechanical properties.

The compositions of the steel wires used were as follows:

Cr-Si steel wire: 0.53% C, 1.58% Si, 0.78% Mn, 0.012% P, 0.012% S, 0.67%Cr, the balance consisting of Fe and impurities.

Cr-V steel wire: 0.51% C, 0.22% Si, 0.81% Mn, 0.011% P, 0.009% S, 0.93%Cr, 0.19% V, the balance con sisting of Fe and impurities.

TAB LE 9 [Variation of Cr-Si steel and Cr-V steel in mechanicalproperties with change in temperature at which the steel is reheated]Heating conditions Reheating conditions Mechanical properties SoakingSoaking Cooling Soaking Soaking Ex. temp. C.) time (min.) conditiontemp. 0.) time (min.) T.S. (kg/mm E. ILA/I.

Air-cooled Remarks: 'I.S., E. and R.A.'1. are as defined in Table 5.

EXAMPLES 31-33 Test pieces of a Cr-Si steel wire having a 4.5-mm.diameter and the same composition as the Cr-Si steel wirebainite-ferrite, when the wire samples heated are cooled :to atemperature immediately above the Ms point thereof.

TABLE 1 [Variation oi meehanical properties of Cr-V steel wire withchange in diameter thereof made by drawing 1t--the Cr-V steel wire beingheat treated by the conventional process] Diameter (min.)

shown in Table 9, were heat treated under the respective difierentconditions except that they were reheated at the same soakingtemperature, as indicated in Table 10. It will be seen from the resultsshown in the table that such a low alloy steel can be provided withexcellent mechanical properties by using a suitable soaking time in theheating and reheating steps even if a soaking temperature in the heatingstep is greatly varied.

TABLE 10 Remarks (l) The Cr-V steel wire used as the starting materialcontained 0.53 C, 0.30 Si, 0.74 Mn, 0.013 P, 0.014 S, 1.07 Cr, 0.20 V,the baalnoe consisting of Fe and incidental impurities.

(2) Some of the test pieces of the starting wire were [Efiects ofheating conditions (soaking temperature and time) on mechanicalproperties of Cr-Si steel] Reheating conditions Heating conditionsSoaking Soaking 1 Soaking Soaking Cooling temp. e '1.S. E. R.A.T. Ex.temp. C.) time (min.) condition 0.) (min.) (kgJmmfl) 31 880 1. 8Air-co3%l5ed to 2. 2 112. 2 11.0 55.3

Remarks: T.S., E. and R.A.T. are as previously defined.

Referring to FIG. 1, curves A, B and C are the cooling rate curves whicheach show that the heat treated Cr-Si steel sample varies in structurewith its diameter or thickness, kind of steel and cooling rate afterheating. Cunve A indicates the formation of a supercooled austenite, Bthe formation of a supercooled austenitebainite and C the formation of asupercooled austenite- TABLE 2 [Chemical compositions of alloy steelswhich may be used in the practice of this invention (percentfl Kind ofalloy steel Si MN P 8 Cr V B Cr-V steel 0. 45-0. 55 0. -0. 0. 65-0. 95Up to 0.035 Up to 0.035 0. 80-1. 10 0.15-1.10 Cr-Si steel 0. -0. 1.20-1. 50 0. 50-0. 80 Up to 0.040 Up to 0.040 0. 8 Bow nstee! 0. 50-0. 600. 150. 35 0.65-0.95 Up to 0.035 Up to 0.035 Si-Mn steei 0. 550. 1.50-2. 20 0. -1. 00 Up to 0.035 Up to 0.035 Mn-Cr steel 0. 50-0. 600.15-0.35 0. 6H. 95 Up to 0.035 Up to 0.035

TABLE 3 [Chemical compositions of alioy steels used in the examples(percentfl Kind of alloy steel 0 Si Mn P S 01' V B Ms point C.)

Cr-V steel 0.50 0. 21 0.77 0. 014 011 318 Cr-Si steel 0. 55 1.37 0. 630. 009 .006 319 Boron steel 0. 57 0. 23 0. 87 0.015 009 298 Si-Mn steel0.61 1. 71 0. 82 0. 018 314 Mn-Cr steel 0. 54 0. 22 0. 76 0. 014 320 TABLE 4 [Heat treating conditions for wires having a larger diameter]Dtiami- Heating conditions Reheating conditions e er 0 wire SoakingSoaking Soaking Soaking Example Kind of steel (mm) temp. 0.) tune (m1n.)Mode of coohng temp. 0.) time (111111.)

1 Cr-V steel 9. 0 920 2. 2 Air-cooled to 455 C. 550 2. 8 2 Cr-Si steel8. 0 875 2.0 Air-cooled to 420 C. 570 2. 5 4 Boron steel 7. 5 870 2. 0Air-cooled to 405 C. 610 2. 5 5 S1-Mn steel 9. 5 920 2. 2 Fureiblycooled to 315 C. 570 2. 8 6 Mn-Cr steel 9. 5 900 2. 5 Air-cooled to 390C. 570 3, 1

TABLE 5 {Variation of mechanical properties of wires with change indiameter thereof by drawing them-the Wires being heat treated accordingto this invention] D.W. (111111.) 9. 0 7. 4 5. 7 4. 1 3. 7 2. 0 R.A.D0%) 0 33. 0 60. 4 79. 3 83. 1 89. 0

(not drawn) Cr-V T.S. (kgJmm. 109.0 128.0 146.0 165. 0 169.0 173. 0 1steel R.A.T. (%J) 46. 5 41.0 38.5 35. 0 34.5 36 0 E. 4. 2 3. 7 3.5 3. 52.5 2.2 V.H. 335 425 445 505 515 530 D.W. (111111.) 8.0 6.4 5.0 4. 2 3.42. 7 R.A.D- 0 36. 0 61. 0 72. 3 82. 0 88. 6

(not 111 wn) E 2 Cr-Si T.S. (kg. mm. 109. 5 123.6 139.6 147. 0 157.5170. 1 steel 11.1111. 58.1 53. 5 es. 2 so. 0 43. 2 s9. 2 E. 0%) 15. 0 5.5 5.3 4. 5 3. 0 2. 0

V.H. 299 379 412 424 439 467 D.W. (111111.) 7. 5 6.0 4. 7 3. 9 3. 2 2. 5RA. 0 35. 1 60. 7 73.0 81.8 88. 9

(not drawn) E 4 Boron T.S. (kgJmm. 107.5 138.8 145. 2 154. 1 163.3 170.5 steel R.A.T. 51. 5 48. 5 48. 7 42. 7 40. 0 E. 13. 0 6. 0 4.3 3. 5 2.32.0 17.11. 320 409 430 448 470 493 D.W. (mm.) 9 5 7. 5 6.0 4. 5 3.9 3. 4R.A.D. (75) 0 37. 7 60. 2 77. 5 83. 2 87. 2

(not drawn) E 5 Si-Mn 'I.S. (kgJmm. 105. 133. 2 142. 7 155.0 162. 5 169.0 51.661 11.111. 46 5 48.5 45.2 45.0 40.8 41.8 E. 15 0 5.0 4.5 4. 3.12.0 V.H. 317 394 425 455 473 497 DW.( 9.5 75 6.0 4.5 3.9 3.4 R A.D. 037. 7 60. 2 77. 5 83. 2 87. 2

(not drawn) E 6 Mil-Cr T S. (kgJmmJ) 99. 2 127.3 7 5 150. 5 160.4 167.4steel R.A.T. 57. 3 50. 0 55. 0 55. 0 45. 0 41. 5 E 14. 5 5. 5 3.8 2.3 2.0 V.H. 288 374 401 433 456 479 Remarks: D.W.:Diameter of wire obtained.

R.A.D.=Reduction of area reached at the end of drawing. T.S.=Te11silestrength. R.A.T.=Reduct1on of area obtained in tension test. 10.:Elongation. V.H.:Vickers hardness.

TABLE 6 [Heat treating conditions for wires having a smaller diameter]Heating conditions Reheating conditions Diameter of Soaking SoakingSoaking Soaking Example Kind of steel wire (min.) temp. 0.) time (min.)Mode of cooling temp. 0.) time (min.)

7 Cr-V steel 4. 5 900 2. 6 Air-cooled to 405 C. 550 3. 3 8 (Jr-Si steel2. 900 0. 8 Air-cooled to 370 C. 600 1. 0 10 Boron steel 5. 6 900 1. 4Air-cooled to 355 C. 580 1. 8 11 Si-Mn steel 3. 2 880 1. 2 Air-coo1ed to365 C. 570 1. 12 Mn-Cr steel 5. 6 880 1. 4 Air-cooled to 380 C. 600 1. 8

TAB LE 7 [Variation of mechanical properties of wires with change indiameter thereof by drawing them-the wires being heat treated accordingto this invention] D.W. (111111.) 4. 5 3. 5 3. 0 2. 85 l. 5 1. 2 R.A.D.0 36.0 55. 0 60. 0 88. 9 92. 8 Ex. 7 Or-V (not drawn) Steel T.S.(kgJmmJ) 105.0 188. 0 147.0 150. 0 198. 2 215. 2 R.A.'1. 53. 5 46. 5 43.5 43. 0 38.5 20. 0 V.H. 340 400 410 415 565 575 D.W. (min.) 2. 0 1. 6 1.3 1. 2 1.1 1. 0 R.A.D. 0 36. 0 57. 7 64. 0 69. 7 75. 0

0t drawn) Ex. 8 Cr-Si T.S. (kg/mm 114. O 140. 6 148. 2 150. 6 156. 5164. 5 steel R.A.T. 32. 0 39.0 28. 0 26. 5 23. 4 21. 1

D.W. 5. 6 3. 6 2. 9 2. 35 1. 9 1. 7 R.A.D 0 58. 6 73. 0 82.0 88. 5 90. 8Ex. 10 Boron ot drawn) steel T.S. (kgJmmfi) 121. 5 158. 0 168. 0 179. 5196. 5 206. 0 3.11.1. 55. 3 50. o 50. o 57. 9 45. s 45. o V.H. 0 476 498500 508 515 D.W. (111111.) 3. 2 2. 75 2.05 1. 65 1. 35 1. 0 R.A.D. 0 26.0 58.9 73. 3 82. 7 90. 8 Ex. 11 Si-Mn ot drawn) Steel T.S. (kg/mm 108. 2136. 0 149. 0 159. 0 168. 4 187. 0 R.A.'I. 51. 5 40. 2 47. 8 55. l 47. 546. 0 V.H. 312 425 465 470 480 486 D.W. (111.111.) 5.6 4.5 3.6 2.9 2. 351.7 R.A.D. 0 25. 5 58. 6 73. 0 82. 0 90. 8 Ex. 12 Mn-Cr (not drawn)steel T.S. (kgJmm 99. 3 124. 0 136. 0 146. 5 157. 5 175. 5 R.A.1. 56. o43. 0 45.8 53. s 55. 1 50. o V.H. 297 382 408 424 432 459 Remarks: D.W.,R.A.D., T.S., R.A.'1., and V.H. are as defined in Table 5.

TABLE 8 [Effects of heat treating conditions] Wireshaving 8-mm. diameterWire so heat treated and then heat treated according to this drawn from8-mm. to 2.7 mm

Heating conditions Reheating conditions invention in diameter (R.A.D.:88.6%)

Soaking Soaking Soaking I Soaking T.S. R.A.T. E. T.S. R.A.T. E Ex temp.0.) time (min.) Cooling temp. 0.) time (min.) (kg/mini) (kgJmmJ) 13 8004 Aitggoslsdc 570 5. 0 113. 3 52. 0 14. 2 179. 2 38. 3 1. 8

o 14 800 6 Air-cooled 570 7. 5 122. 8 49. 2 11. 1 190. 0 81. 4 1. 5

to 360 C 15 875 2 mtr-caogt l ed3 570 2. 5 109. 5 58. 1 15.0 170. 1 39.2 2. 0

o 16 875 4 Aig-crgg ed 570 5.0 122. 1 50. 3 11. 0 189. 7 31. 7 1. 5

o 17 950 1 Aihcggialgta 570 1. 110. 4 57. 7 16. 2 172. 2 30. 0 2. 2

o 18 950 2 Air-cooled 570 2. 5 117. 8 54. 3 12. 8 184. 0 30. 2 2. 0

Remarks: T.S., R.A.T., E., and R.A.D. are as defined in Table 5.

What is claimed is:

1. A process for heat treating a low alloy steel wire, rod, bar or stripmaterial wherein the material to be treated is made of a Cr-V-steelconsisting essentially of 0.45 to 0.55% carbon, 0.15 to 0.35% silicon,0.65 to 0.95% manganese, 0.80 to 1.10% chromium, 0.15 to 0.25% vanadium,the balance consisting of iron and incidental impurities, characterizedby firstly heating the material at a temperature not lower than the Atransformation point there of and keeping it at that temperature for asuitably short time, determined on the basis of 0.12.0 minutes per mm.of the diameter or thickness of the material to be treated; cooling theheated material at a specific cooling rate not specific cooling ratebeing determined according to the kind of steel and the diameter orthickness of the material, to a temperature immediately above the Mspoint thereof; then reheating the cooled material at a tempera ture nothigher than the A transformation point thereby rendering the materialmainly fine-pearlitic in structure, the temperature not lower than the Apoint is 820 C. to 950 C., the temperature above the Ms point is morethan 318 C. to 418 C. and the temperature not higher than the A point is500 to 650 C.

2. A process for heat treating a low alloy steel wire rod, bar or stripmaterial wherein the material to be treated is made of a Cr-Si steelconsitsing essentially of 0.50 to lower than the lower critical coolingrate thereof, the 0.60% carbon, 1.20 to 1.60% silicon, 0.50 to 0.80%manganese, 0.50 to 0.80% chromium, the balance consisting of iron andincidental impurities, characterized by firstly heating the material ata temperature not lower than the A transformation point thereof andkeeping it at that temperature for a suitably short time, determined onthe basis of 0. 12.0 minutes per mm. of the diameter or thickness of thematerial to be treated; cooling the heated material at a specificcooling rate not lower than the: lower critical cooling rate thereof,the specific cooling rate being determined according to the kind ofsteel and the diameter or thickness of the material, to a temperatureimmediately above the Ms point thereof; then reheating the cooledmaterial at a temperature not higher than the A transformation pointthereby rendering the material mainly fine-pearlitic in structure, thetemperature not lower than the A point is 820 C. to 950 C., thetemperature above the Ms point is more than 319 C. to 419 C. and thetemperature not higher than the A point is 500 to 650 C.

3. A process for heat treating a low alloy steel wire rod, bar or stripmaterial wherein the material to be treated is made of a boron steelconsisting essentially of 0.50 to 0.60% carbon, 0.15 to 0.35% silicon,0.65 to 0.95% manganese, 0.65 to 0.95% chromium, not less than 0.0005%boron and the balance consisting of iron and incidental impurities,characterized by firstly heating the material at a temperature not lowerthan the A transformation point thereof and keeping it at thattemperature for a suitably short time, determined on the basis of0.1-2.0 minutes per mm. of the diameter or thickness of the material tobe treated; cooling the heated material at a specific cooling rate notlower than the lower critical cooling rate thereof, the specific coolingrate being determined according to the kind of steel and the diameter orthickness of the material, to a temperature immediately above the Mspoint thereof; then reheating the cooled material at a temperature nothigher than the A transformation point thereby rendering the materialmainly fine-pearlitic in structure, the temperature not lower than the Apoint is 820 C. to 950 C., the temperature above the Ms point is morethan 298 C. to 398 C. and the temperature not higher than the A; pointis 500 to 650 C.

4. A process for heat treating a low alloy steel wire rod, bar or stripmaterial wherein the material to be treated is made of a Si-Mn steelconsisting essentially of 0.55 to 0.65% carbon, 1.50 to 2.20% silicon,0.70 to 1.0% manganese, the balance consisting of iron and incidentalimpurities, characterized by firstly heating the material at atemperature not lower than the A transformation point thereof andkeeping it at that temperature for a suitably short time, determined onthe basis of 0.12.0 minutes per mm. of the diameter or thickness of thematerial to be treated; cooling the heated material at a specificcooling rate not lower than the lower critical cooling rate thereof, thespecific cooling rate being determined according to the kind of steeland the diameter or thickness of the material, to a temperatureimmediately above the Ms point thereof; then reheating the cooledmaterial at a tern perature not higher than the A transformation pointthereby rendering the material mainly fiue-pearlitic in structure, thetemperature not lower than the A point is 820 C. to 950 C., thetemperature above the Ms point is more than 314 C. to 414 C. and thetemperature not higher than the A point is 500 to 650 C.

5. A process for heat treating a low alloy steel 'Wire, rod, bar orstrip material wherein the material to be treated is made of a Mn-Crsteel consisting essentially of 0.50 to 0.60% carbon, 0.15 to 0.35%silicon, 0.65 to 0.95% manganese, 0.65 to 0.95 chromium, the balanceconsisting of iron and incidental impurities, characterized by firstlyheating the material at a temperature not lower than the Atransformation point thereof and keeping it at that temperature for asuitably short time, determined on the basis of 0.1-2.0 minutes per mm.of the diameter or thickness of the material to be treated; cooling theheated material at a specific cooling rate not lower than the lowercritical cooling rate thereof, the specific cooling rate beingdetermined according to the kind of steel and the diameter or thicknessof the material, to a temperature immediately above the Ms pointthereof; then reheating the cooled material at a temperature not higherthan the A transformation point thereby rendering the material mainlyfine-pearlitic in structure, the temperature not lower than the A pointis 820 C. to 950 C., the temperature above the Ms point is more than 320to 420 C. and the tem perature not higher than the A; point is 500 to650 C.

References Cited UNITED STATES PATENTS 3,458,365 7/1969 Nickola et al.148-143 RICHARD O. DEAN, Primary Examiner US. Cl. X.R.

